The objectives of this project are to: 1) determine the clinical significance of a variety of drugs purported to influence the an- ticoagulant response to warfarin but for which only limited data exist (mainly anecdotal experiences), and 2) to determine the extent to which these and other drugs interact with warfarin by mechanisms not previously studied; namely, by influencing the rates of synthesis and/or degradation of the vitamin K dependent clotting proteins. Drugs currently under study or to be studied include: estrogenic and androgenic steroids, clofibrate, and halofenate.